eolas/Programming_Languages/Shell/Passing_arguments_to_scripts.md

categories	tags
Programming Languages	shell

Passing arguments to scripts

Relation between commands and programs

Whenever we issue a command in bash we are really running an executable program that is associated with the command. This is why when we create our own bash scripts we must run chmod to make them executable. When we issue a command like ./file.sh we are running an executable program.

When we run a program like cd or npm we don’t have to type ./cd.sh or ./npm.sh. This is because a reference to the program file is already in our $PATH.

In the case of cd, this is an in-built program and as such it will be sourced from a binary and we have a reference to the binary in path. In the case of npm, this is not an in-built program however in order to run it we must already have it in our PATH.

Passing arguments

A script is really just a function that runs when you source it. As such there needs to be a way for you to pass data to the function so that it can actually act like a function and take arguments. When we use for example cd ./Desktop we are passing a directory name as an argument to the cd program. We can do the same thing with our custom bash scripts.

To pass an argument we simply add the values after the script in the command. For example:

./arguments.sh Thomas 33

We can use built-in variables to return information about the invocation of this script:

#!/bin/bash

echo "File is called $0"
echo "The arguments provided are $@"
echo "The first argument is $1"
echo "The second argument is $2"
echo "Your name is $1 and you are $2 years old"

This outputs:

File is called ./arguments.sh
The arguments provided are Thomas 33
The first argument is Thomas
The second argument is 33
Your name is Thomas and you are 33 years old

Key points:

• $0 designates the script or function that is being executed
• $@ designates a list of all the arguments that are passed to the script
• $1... designates each individual argument